docs: def a_torispherical(D, f, k):
      r'''Calculates depth of a torispherical head according to [1]_.
  
      .. math::
          a = a_1 + a_2
  
      .. math::
          \alpha = \sin^{-1}\frac{1-2k}{2(f-k)}
  
      .. math::
          a_1 = fD(1-\cos\alpha)
  
      .. math::
          a_2 = kD\cos\alpha
  
      Parameters
      ----------
      D : float
          Diameter of the main cylindrical section, [m]
      f : float
          Dish-radius parameter; fD  = dish radius [1/m]
      k : float
          knuckle-radius parameter ; kD = knuckle radius [1/m]
  
      Returns
      -------
      a : float
          Depth of head [m]
  
      Examples
      --------
      Example from [1]_.
  
      >>> a_torispherical(D=96., f=0.9, k=0.2)
      25.684268924767125
  
      References
      ----------
      .. [1] Jones, D. "Calculating Tank Volume." Text. Accessed December 22, 2015.
         http://www.webcalc.com.br/blog/Tank_Volume.PDF'''
      alpha = asin((1-2*k)/(2*(f-k)))
      a1 = f*D*(1 - cos(alpha))
      a2 = k*D*cos(alpha)
      return a1 + a2